1. Field of the Invention
Aspects of the present invention relate to a thin film transistor, and more particularly, to a thin film transistor for a flat panel display and a method of fabricating the same, in which the thin film transistor is capable of performing an on/off switching operation even when a short circuit is formed between contact holes formed on each end portion of a letter U shaped doping region in the thin film transistor.
2. Description of the Related Art
A thin film transistor has widely been used as an on/off switching device in flat panel displays, such as an active matrix organic light emitting diode (AMOLED) or an active matrix liquid crystal display (AMLCD). As a material of the thin film transistor, a thin film of polycrystalline silicon or amorphous silicon, etc., has widely been used. Recently, a crystallizing method using a laser has widely been used. Accordingly, there has been an increased interest in a polycrystalline silicon thin film with excellent current diving capability and rapid operation speed.
The polycrystalline silicon thin film has excellent mobility as compared to the amorphous silicon thin film. The polycrystalline silicon thin film can be formed by integration in a driving circuit on a substrate. Also, the polycrystalline silicon thin film allows implementation of high resolution displays as compared to a typical flat panel display using the amorphous silicon thin film. Thus, it has been recently been spotlighted that the polycrystalline silicon thin film has a tendency to be adopted to a high definition flat panel display such as the AMOLED or an AMLCD.
FIG. 1A is a plan view of a typical thin film transistor, and FIG. 1B is a cross-sectional view taken along a line A-A′ in FIG. 1A. The typical thin film transistor includes a semiconductor layer 10, a doping region 20, a gate electrode 30, and a gate insulating layer 40. The semiconductor layer 10 uses amorphous silicon or polycrystalline silicon. The semiconductor layer 10 is formed as a layer having a predetermined width and length, and is provided with the doping region 20 through a doping of impurities.
The gate insulating layer 40, which is a layer that insulates the gate electrode 30 from the semiconductor layer 10, commonly uses material such as SiNx or SiO2, etc. The gate insulating layer 40 is formed on the semiconductor layer 10, and the gate electrode 30 is formed on the gate insulating layer 40. The gate electrode 30 is provided to supply a voltage and is mainly formed of Mo or MoW, etc. Contact holes (not shown) are formed on each end portion 21 of the doping region 20, wherein each contact hole is provided with source and drain electrodes (not shown). The corresponding active layer (not shown) between the source and drain electrodes becomes a channel region of the thin film transistor.
FIG. 2 is a plan view of a typical thin film transistor when a short circuit occurs. Currently, display products are highly integrated to implement display of high resolution images. In order to minimize the thin film transistor in correspondence with the highly integrated display, the size of the semiconductor layer 10 is formed to be as small as possible, but the width of the end portion 21 (on which the contact holes are to be formed) of the doping region 20 should be formed to be wider than the remainder of the doping region 20.
Generally, the semiconductor layer 10 is formed through a deposition and laser crystallization of the amorphous silicon or the amorphous silicon and a subsequent doping of impurities. However, since the width of the end portion 21 of each doping region 20 is wider as compared to that of the remainder of the doping region 20, there is high possibility of unintended doping of a connection region 50 connecting the end portion 21 of each doping region 20 during the impurities doping process.
If the connection region 50 is doped as explained above, the contact holes are short-circuited and a current always flows therebetween without passing through the thin film transistor. Accordingly, the thin film transistor cannot implement an on/off switching operation. Therefore, the pixels are not normally operated in a region of a display where the short-circuited thin film transistor exists, causing bad lighting problems (pixel defects) such as bright spot (hot pixel), dark spot (dead pixel), and bad clustering (defective pixel clustering), etc.